Vikrti is a Sanskrit word with many meanings. One meaning is variety, variation.
Variety comes in two flavors: planned and in practice. Planned variety means a hatha yoga practice that is intentionally varied from day to day, session to session -- probably not every time, but often enough. Not only asanas can be different, but the form and breathing for them can be different. Variety in practice means spontaneity, but based on knowledge and attentiveness. Variety could be variations in form or anything else, but the lack of advance planning is important. Simply put, get your body out of its rut! As long as sthira (steadiness) and sukha (comfort) are maintained, you can safely (physically & mentally) try anything
The basic concept is this: variety stimulates the brain and body, keeps you fresh. In Desikachar's words, "Varying the asanas renews attention and opens our senses to new experiences." Also variety has a very practical application in asanas, in the physical activities: your body will learn what you do and practice. Its habits, capabilities. If you hold your ankle in only one position during an asana, that is what your body will learn and the capability you will develop in the ankle bones, ligaments, muscles, tendons, blood supply, innervation, etc. In reality you are better off learning that position as your primary position but not neglecting other positions that provide other capabilities for your ankle and thereby your entire body.
Variety can be practiced as long as sthira and sukha are maintained, and as long as the mind can remain focused and calm. Note that sthira and sukha are not automatically attained when a new variation is introduced -- they have to be learned carefully and cautiously -- but there is value in doing so.
Variety can be: intensity of asana (easy through strenuous)
different body parts employed or employed differently
different form, no. of repetitions, duration
different movements into and out of an asana
different sequences of asanas
different breathing pattern
anything else you can think of
Basically as much variety as possible is good, as long as risk of injury is avoided and sthira and sukha are maintained (repetitive, I know, but it is an important point -- variety and spontaneity are different from foolhardiness).
The benefits of vikrti carry over to other aspects of life, in that you accept and embrace different ways of doing things as good. You stay grounded and happy, not off balance and fearful, but able to welcome differences instead of rejecting them.
